New Study Reveals Natural Sediment May Shield Groundwater From Arsenic

A recent study has found that arsenic contamination in deep groundwater from shallower sources may not be as serious as feared, if pumping deep water is limited to domestic use.

Exposure to arsenic-contaminated groundwater has been linked to almost one in every five deaths in Bangladesh, and some 100,000 deep wells have been constructed to pump deeper, cleaner water. Recent modeling studies have suggested that these cleaner water sources are also being contaminated from shallower water seeping down. But the latest study, published in “Nature Geoscience” earlier this week, finds that natural adsorption of arsenic by sediment in aquifers reduces contamination risk in most areas.

“Deep groundwater in Bangladesh is at risk from contamination by arsenic from shallow groundwater seeping downwards if not carefully managed,” said Yan Zheng, who co-authored the study while he was a senior scientist at Columbia University. “The risk is higher if deep groundwater is used for irrigation, which consumes a lot more water than [use for] domestic purposes.”

Zheng said the previous studies suggesting that deep groundwater contamination results from shallower water seeping down to replenish pumped deep water did not consider the influence of sediment, which can adsorb arsenic. They tested this adsorption in Bangladesh and found that sediment removes around 70% of arsenic within a day, reducing the risk of contamination of deep groundwater in most, but not all, areas; and more so when the water is pumped for domestic use only, rather than irrigation. This suggests that current contamination of deep wells is either natural or comes from cases of badly designed wells that allow more seepage, Zheng said.

Zhen recommends that policymakers not use deep groundwater for irrigation and regularly and systematically monitor water quality in areas identified as more vulnerable to contamination.

Wais Kabir, executive chair of the Bangladesh Agricultural Research Council, agreed that irrigation leads to higher risk of arsenic contamination of groundwater and said that Bangladesh needs to “change its food habits” and produce crops that need less irrigation.